Cocompactness and minimizers for inequalities of Hardy–Sobolev type involving N-Laplacian

The paper studies quasilinear elliptic problems in the Sobolev spaces W ^1,p (Ω), ${\Omega\subset{\mathbb R}^N}The paper studies quasilinear elliptic problems in the Sobolev spaces W ^1,p (Ω), W ì \mathbb R^N{\Omega\subset{\mathbb R}^N} , with p = N, that is, the case of Pohozhaev–Trudinger–Moser inequality. Similarly to the case p < N where the loss of compactness in W^1,p(\mathbb R^N){W^{1,p}({\mathbb R}^N)} occurs due to dilation operators u ?t^(N-p)/pu(tx){u {\mapsto}t^{(N-p)/p}u(tx)} , t > 0, and can be accounted for in decompositions of the type of Struwe’s “global compactness” and its later refinements, this paper presents a previously unknown group of isometric operators that leads to loss of compactness in W₀^1,N{W_0^{1,N}} over a ball in \mathbb R^N{{\mathbb R}^N} . We give a one-parameter scale of Hardy–Sobolev functionals, a “p = N”-counterpart of the H?lder interpolation scale, for p > N, between the Hardy functional ò\frac|u|^p|x|^p dx{\int \frac{|u|^p}{|x|^p}\,{\rm d}x} and the Sobolev functional ò|u|^pN/(N-mp)  dx{\int |u|^{pN/(N-mp)} \,{\rm d}x} . Like in the case p < N, these functionals are invariant with respect to the dilation operators above, and the respective concentration-compactness argument yields existence of minimizers for W ^1,N -norms under Hardy–Sobolev constraints.     

Spaces of functions of fractional smoothness on an irregular domain

On an irregular domain G ⊂ ℝ ⁿ of a certain type, we introduce spaces of functions of fractional smoothness s > 0. We prove embedding theorems relating these spaces to the Sobolev spaces W _p ^m (G) and Lebesgue spaces L _p (G).     

Function spaces of Lizorkin-Triebel type on an irregular domain

On an irregular domain G ⊂ ℝ ⁿ of a certain type, we introduce function spaces of fractional smoothness s > 0 that are similar to the Lizorkin-Triebel spaces. We prove embedding theorems that show how these spaces are related to the Sobolev and Lebesgue spaces W _p ^m (G) and L _p (G). Published in Russian in Trudy Matematicheskogo Instituta imeni V.A. Steklova, 2008, Vol. 260, pp. 32–43.     

Boundary trace embedding theorems for variable exponent Sobolev spaces

We study boundary trace embedding theorems for variable exponent Sobolev space W1,p(⋅)(Ω). Let Ω be an open (bounded or unbounded) domain in RN satisfying strong local Lipschitz condition. Under the hypotheses that p∈L^∞(Ω), 1?infp(x)?supp(x)<N, |∇p|∈Lγ(⋅)(Ω), where γ∈L^∞(Ω) and infγ(x)>N, we prove that there is a continuous boundary trace embedding W1,p(⋅)(Ω)→Lq(⋅)(∂Ω) provided q(⋅), a measurable function on ∂Ω, satisfies condition for x∈∂Ω.     

Exponential decay and Fredholm properties in second-order quasilinear elliptic systems

We consider second-order quasilinear elliptic systems on unbounded domains in the setting of Sobolev spaces. We complete our earlier work on the Fredholm and properness properties of the associated differential operators by giving verifiable conditions for the linearization to be Fredholm of index zero. This opens the way to using the degree for C¹-Fredholm maps of index zero as a tool in the study of such quasilinear systems. Our work also enables us to check the Fredholm assumption which plays an important role in Rabier's approach to proving exponential decay to zero at infinity of solutions.     

Infinitely many solutions for elliptic problems with variable exponent and nonlinear boundary conditions

We analyze a class of quasilinear elliptic problems involving a p(·)-Laplace-type operator on a bounded domain W ì \mathbb R^N{\Omega\subset{\mathbb R}^N}, N ≥ 2, and we deal with nonlinear conditions on the boundary. Working on the variable exponent Lebesgue–Sobolev spaces, we follow the steps described by the “fountain theorem” and we establish the existence of a sequence of weak solutions.     

Riesz potential and Sobolev embeddings on generalized Lebesgue and Sobolev spaces Lp(·) and Wk,p(·)

We study the Riesz potentials I_αf on the generalized Lebesgue spaces L^p(·)(?^d), where 0 < α < d and I_αf(x) ? ∫equation/tex2gif-inf-3.gif |f(y)| |x – y|^{α – d} dy. Under the assumptions that p locally satisfies |p(x) – p(x)| ≤ C/(– ln |x – y|) and is constant outside some large ball, we prove that I_α : L^p(·)(?^d) → L^p?(·)(?^d), where . If p is given only on a bounded domain Ω with Lipschitz boundary we show how to extend p to on ?^d such that there exists a bounded linear extension operator ? : W^1,p(·)(Ω) ? (?^d), while the bounds and the continuity condition of p are preserved. As an application of Riesz potentials we prove the optimal Sobolev embeddings W^k,p(·)(?^d) ?L^p*(·)(R^d) with and W^1,p(·)(Ω) ? L^p*(·)(Ω) for k = 1. We show compactness of the embeddings W^1,p(·)(Ω) ? L^q(·)(Ω), whenever q(x) ≤ p*(x) – ε for some ε > 0. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compact Sobolev embedding theorems involving symmetry and its application

Given a bounded regular domain with cylindrical symmetry, functions having such symmetry and belonging to W ^1,p can be embedded compactly into some weighted L ^q spaces, with q superior to the critical Sobolev exponent. A similar result is also obtained for variable exponent Sobolev space W ^1,p(x). Furthermore, we give a simple application to the p(x)-Laplacian problem.     

Spectral Approximation and Index for Convolution Type Operators on Cones on L^{p}(\mathbb {R}^2)

We consider an algebra of operator sequences containing, among others, the approximation sequences to convolution type operators on cones acting on L^p(\mathbb R²)L^{p}(\mathbb {R}^2), with 1 < p < ∞. To each operator sequence (A_n) we associate a family of operators W_x(A_n) ? L(L^p(\mathbb R²))W_{x}(A_{n}) \in \mathcal {L}(L^{p}(\mathbb {R}^2)) parametrized by x in some index set. When all W_x(A_n) are Fredholm, the so-called approximation numbers of A_n have the α-splitting property with α being the sum of the kernel dimensions of W_x(A_n). Moreover, the sum of the indices of W_x(A_n) is zero. We also show that the index of some composed convolution-like operators is zero. Results on the convergence of the e\epsilon-pseudospectrum, norms of inverses and condition numbers are also obtained.     

The hardy-littlewood maximal function of a sobolev function

We prove that the Hardy-Littlewood maximal operator is bounded in the Sobolev spaceW ^1,p (R ⁿ ) for 1<p≤∞. As an application we study a weak type inequality for the Sobolev capacity. We also prove that the Hardy-Littlewood maximal function of a Sobolev function is quasi-continuous.     

On the stability of the index of unbounded nonlocal operators in Sobolev spaces

Unbounded operators corresponding to nonlocal elliptic problems on a bounded region G ⊂ ℝ² are considered. The domain of these operators consists of functions in the Sobolev space W ₂ ^m (G) that are generalized solutions of the corresponding elliptic equation of order 2m with the right-hand side in L ₂(G) and satisfy homogeneous nonlocal boundary conditions. It is known that such unbounded operators have the Fredholm property. It is proved that lower order terms in the differential equation do not affect the index of the operator. Conditions under which nonlocal perturbations on the boundary do not change the index are also formulated. Published in Russian in Trudy Matematicheskogo Instituta imeni V.A. Steklova, 2006, Vol. 255, pp. 116–135.     

Pseudodifference Operators on Weighted Spaces, and Applications to Discrete Schrödinger Operators

We study pseudodifference operators on Z ^N with symbols which are bounded on Z ^N ×T ^N together with their derivatives with respect to the second variable. In the same way as partial differential operators on R ^N are included in an algebra of pseudodifferential operators, difference operators on Z ^N are included in an algebra of pseudodifference operators. Particular attention is paid to the Fredholm properties of pseudodifference operators on general exponentially weighted spaces l _w ^p (Z ^N ) and to Phragmen–Lindelöf type theorems on the exponential decay at infinity of solutions to pseudodifference equations. The results are applied to describe the essential spectrum of discrete Schrödinger operators and the decay of their eigenfunctions at infinity.     

Density of smooth maps in Wk, p(M, N) for a close to critical domain dimension

Assuming m − 1 < kp < m, we prove that the space C ^∞(M, N) of smooth mappings between compact Riemannian manifolds M, N (m = dim M) is dense in the Sobolev space W ^k,p (M, N) if and only if π _m−1(N) = {0}. If π _m−1(N) ≠ {0}, then every mapping in W ^k,p (M, N) can still be approximated by mappings M → N which are smooth except in finitely many points.     

On Transference of Multipliers on Matrix Weighted <Emphasis Type="Italic">L</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>-Spaces

We consider a periodic matrix weight W defined on ℝ ^d and taking values in the N×N positive-definite matrices. For such weights, we prove transference results between multiplier operators on L _p (ℝ ^d ;W) and L_p(\mathbb T^d;W)L_{p}(\mathbb {T}^{d};W), 1<p<∞, respectively. As a specific application, we study transference results for homogeneous multipliers of degree zero.     

Approximation Numbers of Sobolev Embedding Operators on an Interval

Consider the Sobolev embedding operator from the space of functionsin W^1,p(I) with average zero into L^p, where I is a finite intervaland p>1. This operator plays an important role in recentwork. The operator norm and its approximation numbers in closedform are calculated. The closed form of the norm and approximationnumbers of several similar Sobolev embedding operators on afinite interval have recently been found. It is proved in thepaper that most of these operator norms and approximation numberson a finite interval are the same.     

Generalized Solutions of Nonlocal Elliptic Problems

An elliptic equation of order 2m with general nonlocal boundary-value conditions, in a plane bounded domain G with piecewise smooth boundary, is considered. Generalized solutions belonging to the Sobolev space W ₂ ^m (G) are studied. The Fredholm property of the unbounded operator (corresponding to the elliptic equation) acting on L ₂(G), and defined for functions from the space W ₂ ^m (G) that satisfy homogeneous nonlocal conditions, is established.__________Translated from Matematicheskie Zametki, vol. 77, no. 5, 2005, pp. 665–682.Original Russian Text Copyright ©2005 by P. L. Gurevich.     

The spectral remainder estimates for non-isotropic polyharmonic operators on bounded and quasibounded domains

Summary Approximation numbers of the embedding operator of the nonisotropic Sobolev space W^(m), 2(Q) into L²(Q), Q being an n-dimensional open box, are applied to the study of spectral asymptotics. We deal with the nonisotropic polyharmonic operators on bounded and quasibounded open sets in Rⁿ. The quasibounded open sets are required to satisfy theG _k,a ^(m),2 -condition. Not only the leading terms of the asymptotic formulae but also remainder terms are obtained. An application is made to a quasibounded domain whose width has a negative power-type decay at infinity.     

von Neumann��s mean ergodic theorem on complete random inner product modules

We first prove two forms of von Neumann’s mean ergodic theorems under the framework of complete random inner product modules. As applications, we obtain two conditional mean ergodic convergence theorems for random isometric operators which are defined on L _ℱ ^p (ℰ, H) and generated by measure-preserving transformations on Ω, where H is a Hilbert space, L ^p (ℰ, H) (1 ⩽ p < ∞) the Banach space of equivalence classes of H-valued p-integrable random variables defined on a probability space (Ω, ℰ, P), F a sub σ-algebra of ℰ, and L _ℱ ^p (ℰ(E,H) the complete random normed module generated by L ^p (ℰ, H).     

Topology of Sobolev mappings III

We establish a necessary and sufficient topological condition for maps that are in W^1,p(M, N) to be connected by continuous paths in W^1,p(M, N) to maps in W^1,q(M, N), q > q ≥ 1. We also obtain a necessary and sufficient topological condition under which W^1,q(M, N) is strongly dense in W^1,p(M, N). Several results concerning sequential weak density of smooth (or W^1,q(M, N) maps in W^1,p(M, N) are also proven. © 2003 Wiley Periodicals, Inc.